Methods for the detecting or measuring the content of gaseous carbon dioxide (CO.sub.2) in a mixture of gases utilizing chemical absorption belongs to the well-known technique.
German Patents Nos 919510 and 1 007 525 both discloses selective absorption of CO.sub.2 on a substance which contains a pH-sensitive dye. The change in the pH-value caused by the CO.sub.2 bound to the substance becomes apparent as a change in colour of the dye which is present in the substance. Accordingly the change in colour becomes a measure of the content of CO.sub.2 in the gas flow under investigation.
The problems encountered with these early systems are that the absorbing surface must be kept in hermetically closed glass tubes and that, once said surface gets into contact with the gas flow, an irreversible reaction is obtained, i.e. the device becomes unusable after having been exposed to the test gas. Thus it cannot be used e.g. for monitoring CO.sub.2 concentrations which vary with time.
A reverible CO.sub.2 indicator device is disclosed in U.S. Pat. No. 4,728,499. This prior art device comprises a system consisting of a pH-sensitive indicator dye, a basic substance and a viscous hygroscopic liquid. The indicator is able both to absorb and desorb CO.sub.2 with a time constant of a few seconds so that it can be used e.g. in hospitals for monitoring the breathing of a patient. During exhalation, which takes about 4 seconds, the air contains 3-5 % by volume of CO.sub.2 whereas the CO.sub.2 content during inhalation, which typically takes about 2 seconds, is about 0.05% by volume corresponding to the normal concentration of CO.sub.2 in atmospheric air. The indicator hence changes colour to and fro with breathing about 10 times a minute.
The main disadvantage of this known system is that it is strongly hygroscopic and hence the indicator will absorb water vapour from the gas under investigation ultimately resulting in that the system ceases to respond to CO.sub.2. In view of the fact that exhaled air is saturated with water vapour at about 30.degree. C. this indicator will only be able to monitor the breathing of a patient for a few minutes.
Another disadvantage exhibited by this prior art device is that the indicator must be stored in a hermetically closed, absolutely dry environment free from carbon dioxide prior to use.
Another type of a reversible calorimetric device is disclosed by U.S. Pat. No. 5,005,572 and WO91/05252 and an article in Analytical Chemistry, Vol 64, page 1383 (1992).
These systems are based on pH-sensitive indicator dyes, 30 water-insoluble organic quarternary (e.g. ammonium or phosphonium) hydroxides as a basic substance and additional substances in order to facilitate the absorption/desorption of CO.sub.2.
These known indicator devices appear to function reversibly for several days and they are indicating satisfactory both in humid and dry environments.
The major disadvantage of these devices is that the strong base decomposes with time so that the indicator slowly becomes permanently "acid" and accordingly useless. Then it exhibits the "acid" colour all the time as if it were exposed to a constantly high concentration of CO.sub.2.
This decomposition of the base is strongly depending on the temperature of the environment and hence the useful length of life of an indicator device which has been stored while awaiting use is difficult to predict and check.
It is an object of the present invention to provide a method of increasing the shelf life of a reversible calorimetric device for indicating carbon dioxide.
It is another object of the present invention to provide a package of a reversible colorimetric device having increased shelf life.